Patent Document 1 discloses a method for producing a vinylidene fluoride based polymer aqueous dispersion having a solid matters content of 30 to 60 wt. % and a small particle size, i.e., an average particle size of 200 nm or lower, which is suitable for coating materials, wherein the following compounds are effectively used as fluorine based reactive emulsifiers:
Compound 1: F2C═CF(CF2)1-10 COOM
Compound 2: F2C═CF(CF2CFX)1-5COOM
Compound 3: F2C═CFO(CF2)1-10COOM
Compound 4: F2C═CFO(CF2CFXO)1-10CF2CF2COOM
Compound 5: H2C═CFCF2O[CF(CF3)CF2O]0-10CF(CF3)COOM
Compound 6: F2C═CFCF2O[CF(CF3)CF2O]1-10CF(CF3)COOM                X: F or CF3 group        M: H, NH4, or alkali metal        
Compound 1 is synthesized by dechlorination of dichloro-perfluoroalkyl carboxylic acid obtained by heating ω-hydro-α,β-dichloroperfluoroalkane at a high temperature in the presence of chlorine or nitrogen oxide. Not only the raw material is difficult to obtain, but also high temperature oxidation is required. Thus, this method is not practical (Non-Patent Document 1).
The method for producing Compound 4 is described in detail in Patent Document 2. The use of sulfur trioxide or tin chloride to obtain alkoxycarbonyl carbonyl fluoride (ROOCCF2COF), which is a starting material, causes a safety problem in the reaction process. Moreover, corrosion-resistant equipment is required; thus, special equipment designs are required.
Compound 5 is obtained using, as a starting material, tetrafluorooxetane, which is obtained by the reaction of tetrafluoroethylene with formaldehyde as shown below, according to the following scheme. However, the process is long, and hydrogen fluoride, and zinc metal used in the dehalogenation reaction cause waste problems. Thus, this method is not industrially advantageous (Patent Document 3).

The details of the method for producing Compound 6 of the formula: CF2═CFCF2OCF(CF3)CF2OCF(CF3)COOH are unclear. Furthermore, Patent Document 3 discloses a method for obtaining carboxylic acid ammonium and carboxylic acid alkali metal salts via carboxylic acid. However, perfluorovinyl ether is often polymerized even in the presence of acid, such as carboxylic acid, and converted into a polymer. As a result, the yield is reduced. Overall, this method is not suitable as a method for obtaining carboxylic acid metal salts.